Basic Planetary Gear Set
A planetary gear set contains three gears that remain in mesh at all times. The transmission can change the gear ratio by driving one member of the gear set while holding another. This causes the third member of the gearset to be the driven gear.
Sun Gear: The sun gear, similar to the sun in our solar system, is located at the center of the planetary gear set. This gear remains in constant mesh with the pinion gears. Please don't confuse the pinion or planet carrier with the planetary gear set; it is a member of the planetary gearset.
Ring Gear: The ring/annulus gear has internal teeth and remains in constant mesh with the pinion gears. It's the outer gear of the set and remains in constant mesh with the pinion gears at all times.
Pinion Gears: The pinion carrier is located between the ring and sun gear. It remains in constant mesh with both the sun and the ring gear. It consists of several pinion gears contained inside of a carrier called the planet carrier. This carrier contains the needle bearings and pins that the pinion gears ride on. As mentioned earlier, the pinion carrier's planet gears remain in mesh with the sun gear as well as the ring gear at all times.